1. Field of the Invention
The present invention relates to a technique to control the exhaust state for a fuel cell for a mobile unit.
2. Description of the Related Art
In fuel cells, a reaction generates electricity and discharges a corresponding amount of generated water. Especially in fuel cells for automobiles, a larger amount of water is generated as the vehicle travels a longer distance. Most fuel cells for automobiles are of a solid polymer type, which basically operates at low temperatures. A common problem with the fuel cells is the treatment of the generated water, that is, preventing the generated water from freezing on the road in cold regions or from splashing toward any following vehicles.
Depending on the ambient air conditions and the driving conditions, however, it is also necessary to suppress the generation of white smoke from the exhaust port of a discharge passage for off gas. Generation of the white smoke is not desirable from the standpoint of the merchantability of the automobiles. In some instances, the impact of the automobiles on the surroundings due to the white smoke should be taken into account. In view of the above, proposals have been made to suppress the white smoke in fuel cells for vehicles in Japanese Patent Application Publication No. 7-169498 (JP-A-7-169498) and Japanese Patent Application Publication No. 2001-185199 (JP-A-2001-185199).
To reduce the white smoke, in general, the fuel cells for vehicles have adopted means for cooling or heating the off gas beyond the temperature range where white smoke is easily generated, means for suppressing the amount of scavenged air, and so forth. Therefore, energy is required to reduce the white smoke, which contradicts the requirement to improve the power generation efficiency.
That is, the fuel cells described in the above documents estimate the generation of white smoke mainly based on the difference between the exhaust air temperature and the ambient temperature. Then, a white smoke reduction process is executed according to the estimation results. If the white smoke reduction process is executed frequently, the fuel efficiency is reduced due to the heating, the energy efficiency is reduced, the output is reduced due to the suppressed amount of scavenged air, etc., to a larger degree.
As a result of a more detailed examination of the circumstances where the white smoke is generated, the following may be pointed out. The white smoke generated is not noticeable while the vehicle is in motion because of the diffusion effect of the head wind, but is noticeable while the vehicle is stationary or traveling at low speeds. The visibility of the white smoke varies in accordance with the environmental conditions around the vehicle, such as whether it is daytime or nighttime and whether it is sunny or rainy. The white smoke generated affects the vision of the operator when backing the vehicle.